The present invention relates to a chemical etching method which is applicable, for instance, to the manufacture of printed circuits.
Sometimes a spiral coil is produced by forming, through chemical etching, fine conductor leads on an insulating film e.g. conductor leads 30 .mu.m to several mm wide and 10 to 200 .mu.m thick with a spacing 0.6 to 1.5 times larger than the thickness as set forth, for example, in U.S. Pat. No. 4,340,833 "Miniature Motor Coil" issued on July 20, 1982. That is to say, fine conductor leads sometimes are formed by chemical etching with a spacing substantially equal to the thickness of the conductor layer. In chemical etching under such conditions and with close dimensional tolerances one encounters the problem of what is called side etching. More particularly, in forming a pattern of required shape by chemical etching, an etching-resistant mask layer of a pattern corresponding to the required shape is formed on a member to be etched and an etchant is supplied to the portional areas of the member not covered with the mask layer for etching. If the etching were effected only in a direction perpendicular to the surface of the member to be etched, the pattern of the required shape would be obtained but, in practice, as the etching proceeds, the portions underlying the mask layer are also etched in a direction parallel to the surface of the member to be etched. The unwanted etching of the portions underlying the mask layer is called side etching. The occurrence of such side etching makes it difficult to achieve deep etching at small intervals with high accuracy.
To obviate the problem of side etching, it is general practice in the prior art to carry out etching a plurality of times and to form an etching-resistant layer on the side wall of a side-etched portion of the member to be etched (hereinafter referred to as the side-etched wall surface), thereby preventing further progress of side etching. According to one conventional method, etching is stopped after having proceeded to a predetermined depth and the portion of the mask layer which extends outwardly from the unetched portion of the member over the side etched space, is made molten by heating and brought into contact with the side-etched wall surface, forming therein the etching-resistant layer. With this method, however, when the mask layer is made molten by heating, its extended portion resulting from side etching hangs down under the influence of gravity onto the side-etched wall surface and, at the same time, it is pulled by the cohesive force to the top surface of the member to be etched, so that the etching-resistant layer cannot be formed over the entire area of the side-etched wall surface.
According to another known method of forming the etching-resistant layer on the side-etched wall surface, the member to be etched, after being etched to a predetermined depth, is dipped into a swelling liquid to swell the mask layer whereafter it is dried by heating. In this case, when the swelling liquid remaining on the side-etched wall surface is being evaporated, the extended portion of the mask layer is gradually pulled to the side-etched wall surface by the surface tension acting on the remaining swelling liquid. With this method, the extended portion of the mask layer is bent down into contact with the side-etched wall surface but, in practice, it does not closely contact the side-etched wall surface; in particular, it is difficult to form end portions and corners of a required pattern with high accuracy.